The geology of petroleum traps

There are two basic types of petroleum traps – structural and stratigraphic.

Structural traps occur when reservoir rock layers are deformed, such as in a fault or a fold. In a fault trap, a rift occurs in the rock, causing the layers to slip and tilt in one direction or another at the point of deformation, meaning that the migration of oil through a particular layer is halted. There are two types of folds – synclines and anticlines. Of these, normally only anticlines become petroleum traps. An anticline occurs when rock layers form an arch-like structure and the petroleum migrates to the highest point and is prevented from moving further by an overlying bed of impenetrable rock.

Stratigraphic traps can be formed by the deposition of reservoir rock, by an angular unconformity, or by a change in the porosity of the sedimentary layer. Examples of depositions include a river channel sandstone or a limestone reef encased in shale. One example of an angular unconformity is when an angular sedimentary layer is truncated, or cut off, by a horizontal layer. This is known as a truncation. Another, which is called a pinch-out, is whe^re the sedimentary layer is trapped between two other layers when they meet at a point.

Petroleum reservoirs can also be created by traps which are both structural and stratigraphic. These are called combination traps. An example of this is a salt dome, where a layer of salt pushes upwards and deforms the surrounding layers.

Task 2. Read this dialogue and answer questions. Act out the dialogue:

Задание 2. Прочитайте диалог и ответьте на вопросы. Разыграйте диалог по ролям:

Manager: So, can you describe the geological formation of Carson Sound?

Geophysicist: Sure. As we have explained in our report, this field appears to be an extension of the Prudhoe Bay field. If I could describe the geology of Prudhoe Bay first of all, it will help me to explain the geology of Carson Sound. This diagram shows the central North Slope. As you can see, Prudhoe Bay is the result of a combination of structural and stratigraphic circumstances. They were formed by unconformities created when the rocks were uplifted during rifting and then buried again.

Manager: So, how is the accumulation trapped?

Geophysicist: Well, on the north side of the bay the field is sealed by a normal fault, and in the south by south-dipping shales, which overlie the reservoir sand. The western extent of the oil is limited by a structural saddle in the upper surface of the reservoir. Unconformities and overlaying shales seal the crest and the eastern flank of the oilfield.

Manager: So Carson Sound is part of the same formation, is it?

Geophysicist: Well, the oil is certainly from the same source, but there are one or two crucial differences.

Вопросы:

Questions:

1 Why does the geophysicist describe the geology of Prudhoe Bay?

2 How is the field sealed on each side – north, south, west and east?

Task 3. Now read the continuation of the meeting and discuss the questions in pairs:

Задание 3. Прочитайте продолжение диалога и обсудите следующие вопросы в парах:

1. What is the connection between Mukluk and Carson Sound?

2. Do you think that the manager should be worried?

Geophysicist: I have to tell you that there is always the risk that Carson Sound could turn out to be a Mukluk.

Manager: What do you mean, a Mukluk?

Geophysicist: Yes, I suppose that was before your time! Mukluk was one of the biggest financial disasters in oil and gas history – in 1983, nearly twenty years after the huge success story of Prudhoe Bay.

Manager: Go on.

Geophysicist: Well, Mukluk is just on the other side of Prudhoe Bay, and it was also an angular unconformity trap, like Carson Sound. Seismic data indicated a potential reservoir rock in the same Sadlerochit Sandstone with a shale cap rock. The lease was sold for an estimated $1.5 billion and 12 companies spent $100 million on building an artificial island in the Beaufort Sea, and then a further $150 million on exploration drilling. They were looking at anything between two billion and 10 billion barrels of oil, depending on who you believe.

Manager: Wow! So what happened?

Geophysicist: Absolutely nothing. It turned out that the oil had leaked out of the formation centuries ago. There was nothing left. It is still the most expensive dry hole ever!

Manager: Well, this isn’t nearly as big, is it?

Geophysicist: No, but you still don’t want to waste your money, do you?

Manager: And what are the chances of that happening?

Task 4. Read the descriptions of each stage of the process below which explain how a petroleum trap is formed in the two largest fields in North America, the East Texas field and the Prudhoe Bay field. Put the first five sentences in the correct chronological order. The final stage in the last sentence is in the correct place:

Задание 4. Прочитайте описание стадий формирования нефтяных ловушек в месторождениях East Texas и Prudhoe Bay. Распределите пять предложений в хронологическом порядке: